Liquid crystalline phases of lipids are known in the prior art. These phases are called liquid crystalline since they have degrees of order which are intermediate between the three dimensional order of a crystal and the random distribution of a liquid. These phases have order in one or two dimensions characterized by onionskin or lamellar arrangements of the lipids when dispersed in water.
When liquid crystals are subjected to energy in the form of ultrasonic radiation, they can be broken down to single layer vessicles of small dimensions called liposomes.
In recent years encapsulation of various medicaments in phospholipid-chloesterol liposomes has been accomplished. The systems described are ternary systems of a phospholipid plus cholesterol and water. Basically, these systems are prepared by dissolving the phospholipid and cholesterol in a solvent which is evaporated to leave a thin film of lipid. The aqueous phase medicament is then added, which swells the cholesterol phospholipid mixture to encapsulate the medicament. Subsequent ultrasonic irradiation provides the liposomes.
Vanlerberghe et al. U.S. Pat. No. 4,217,344, discloses the preparation of dispersions of liposomes containing an active substance, which after size reduction by ultrasonic treatment, are separated from the dispersion by gel filtration.
Preparation of phospholipid-sterol liposomes by gel filtration without sonication has been described by J. Brunner et al. "Single Bilayer Vesicles Prepared Without Sonication Physico-Chemical Properties" Biochemica et Biophysica Acta, 455 (1976) 322-331.
Basically, the prior art preparation of phospholipid-sterol liposomes using gel filtration involved two major steps:
(1) The preparation of the liposome in the traditional fashion by,
(a) Dissolving the lipid substances in organic solvents to evenly disperse the components, and then evaporating the solvent. PA1 (b) Adding water or other aqueous media containing drug to the lipid film to form the liposomes.
The liposome dispersion is then sonicated or not sonicated depending on the type of liposome desired.
(2) The liposome preparation is passed over a gel-filtration column to remove untrapped drug and to isolate liposomes of a uniform size.
In contrast, the present liposome process using gel filtration involves the following steps:
(1) Solubilization and homogeneous dispersion of drug and lipid components in a detergent (rather than organic solvent).
(2) Passage of the detergent solution over a gel-filtration column which removes the detergent, causing the formation of the liposome.
In the present invention the concentration of components may be in excess of that necessary for liposome formation. The liposome formed will be what is physically the most stable form. The present procedure may be made limiting by initially adjusting the concentration of the components. Thus, the present procedure allows for greater control over the final liposome composition than the prior art procedure.